Wednesday, 13 November 2013

Security issues with using PHP's escapeshellarg

Using user supplied data on the command line is traditionally a security
disaster waiting to happen. In an infinite universe there are however times
when you might need to do just that. You will be glad to know that PHP provides
two functions to aid you with security in those situations: escapeshellcmd and
escapeshellarg.The PHP documentation defines these functions as:

escapeshellcmd() escapes any characters in a string that might be used
to trick a shell command into executing arbitrary commands. This function
should be used to make sure that any data coming from user input is escaped before
this data is passed to the exec() or system() functions, or to the backtick
operator.Following characters are preceded by a backslash:
#&;`|*?~<>^()[]{}$\, \x0A and \xFF. ' and " are escaped only if
they are not paired. In Windows, all these characters plus % are replaced by a
space instead.

escapeshellarg() adds single quotes around a string and quotes/escapes
any existing single quotes allowing you to pass a string directly to a shell
function and having it be treated as a single safe argument. This function
should be used to escape individual arguments to shell functions coming from
user input. The shell functions include exec(), system() and the backtick
operator.

There are some caveats around the use of these functions which the
documentation doesn't cover, command line switches inside single quotes are
still treated as command line switches. For example: ls '--help' will print the
help text for the ls command. Thus it may be possible to inject data to alter
the intended execution, typically referred to as command injection. In order to
illustrate this bug I have created a simple proof of concept script which will
spawn a bind shell on port 4444 by diverting the execution of tar with command
line switches:

The response from the PHP security team is that
this is expected behavior, and that it is not possible to protect programs that use parameters in unsafe ways. While I understand their point of view, I still feel
that the documentation does not clearly highlight the potential risk around
using escapeshellarg. And if you are doing source code reviews I would take a
closer look at any operation which relies on escapeshellarg to sanitise user
supplied input.